1. Field of the Invention
The present invention relates to homeotropically aligned liquid crystal films, a process for producing the same and uses of the liquid crystal films.
2. Description of the Related Art
In recent years, it has been proposed to apply polarizing plates or phase plates using liquid crystals having photopolymerizablity to optical elements. Such optical elements are obtained by polymerizing polymerizable liquid crystals having optical anisotropy in a liquid crystal state to fix alignment of the liquid crystals. After the polymerizable liquid crystals are subjected to proper alignment control in the liquid crystal state, they can be polymerized with keeping their alignment state. Therefore, by fixing the alignment vector of the liquid crystal skeleton in an alignment state, such as homogeneous alignment (horizontal alignment), tilt alignment (inclined alignment), homeotropic alignment (vertical alignment) or twisted alignment (distorted alignment), polymers having various optical anisotropies can be obtained. The expression “exhibiting such alignment as above” is sometimes referred to as “having homogeneous alignment”, “having tilt alignment”, “having homeotropic alignment” or “having twisted alignment” simply hereinafter.
In the polymer having homeotropic alignment, the direction of the optical axis is nz direction, and the refractive index in the direction of the optical axis is larger than that in the direction crossing the direction of the optical axis at right angles, so that in the index ellipsoid, the polymer is classified into positive C-plate. By combining the positive C-plate with a film having another optical function, it can be applied to optical compensation of horizontally aligned liquid crystal mode, i.e., so-called IPS (in-plane switching) mode, for example, improvement in viewing angle properties of polarizing plates (see, for example, M. S. Park, et al., IDW'04 FMC8-4; M. Nakata, et al., SID'06 P-58; and WO 05/38517A1).
As a method to form such homeotropic alignment, there has been proposed, for example, a method of applying a surface active agent having a long-chain alkyl group, a coupling agent, a metal complex or the like onto a substrate such as a glass substrate or a method of using a side chain type liquid crystal polymer (see, for example, Quarterly Elements of Chemistry, No. 22 “Chemistry of Liquid Crystals”, edited by The Chemical Society of Japan, pp. 99-100 (1994); Japanese Patent Laid-Open Publication No. 319408/1998; Japanese Patent Laid-Open Publication No. 240890/1999; Japanese Patent Laid-Open Publication No. 365635/2002; and Japanese Patent Laid-Open Publication No. 2927/2003).
In the conventional methods, however, there reside various problems such that uniformity or stability of the homeotropic alignment is insufficient, the number of steps required for film production is increased, and coating failures such as repelling and pinhole take place when the polymerizable liquid crystal is applied. Further, there is another problem that if a polyimide based polymer film for vertical alignment, which is used for a liquid crystal device, is formed on a substrate such as a glass substrate and baked at a temperature of about 200° C. (to promote imidization), it is impossible to apply a polymerizable liquid crystal composition onto the resulting baked surface. In addition, there is another problem that if an optical plastic film such as a film of TAC (triacetyl cellulose) or a norbornene resin is selected as a substrate, the film cannot be used under the aforesaid baking temperature conditions from the viewpoint of heat resistance.
The present inventors have found so far that a polymerizable liquid crystal film having homeotropic alignment can be obtained by irradiating a polymerizable liquid crystal layer composed of a polymerizable liquid crystal composition containing an acrylate based polymerizable liquid crystal compound and an organic silicon compound having primary amine with ultraviolet rays in an inert atmosphere and thereby curing the layer (see Japanese Patent Laid-Open Publication No. 126757/2006) In this method, however, there reside problems of complicated production process and high cost because it is necessary to form an inert atmosphere in the curing stage.
In order to solve the above problems, the present inventors have found a polymerizable liquid crystal composition containing a liquid crystal compound having a polymerizable epoxy (cyclic ether) group (see Japanese Patent Laid-Open Publication No. 23656/2006 and Japanese Patent Laid-Open Publication No. 23657/2006). In case of polymerizable liquid crystal compounds having the same polymerizable groups as each other, however, sufficient curability is not obtained occasionally, or the cost becomes high because of specific structures of the compounds.